1. Field of the Invention
The present invention relates to drilling and invert emulsion fluids utilized in the drilling, completion and workover of subterranean wells and in particular oil and gas wells.
2. Background
Many types of fluids have been used in the drilling of oil and gas wells. The selection of an oil based drilling fluid involves a careful balance of the both the good and bad characteristics of such fluids. In a particular application the type of well to be drilled and the geological characteristics of the oil or gas field in which the well is to be drilled, the location of the well, and the like all affect the decision as to the type of drilling fluids used.
For a number of years, oil based drilling fluids and invert emulsion muds have been utilized in the drilling of subterranean wells. These fluids typically are comprised of a hydrocarbon oil or mixtures of oils as the major component of the liquid phase to which various materials are added to impart the desired drilling fluid properties. These fluids are well adapted for use in underground formations containing water sensitive clays or shales which swell and disintegrate when contacted by water based drilling fluids. True oil-based fluids are substantially free of water, lose mostly oil as the filtrate and cause no swelling or disintegration of water sensitive clays and shales. Water-in-oil emulsions are also used as drilling fluids. These fluids contain oil as the continuous phase and water as the external or discontinuous phase. A water-in-oil emulsion is also known as an invert emulsion. The true oil-based and the invert emulsion muds generally contain one or more weighting agents, surfactants, viscosifiers, fluid loss control agents or bridging agents.
The primary benefits of selecting an oil based drilling fluid include: superior hole stability, especially in shale formations; formation of a thinner filter cake than the filter cake achieved with a water based mud; excellent lubrication of the drilling string and downhole tools; penetration of salt beds without sloughing or enlargement of the hole as well as other benefits that should be known to one of skill in the art.
An especially beneficial property of oil based drilling fluids are their excellent lubrication qualities. These lubrication properties permit the drilling of wells having a significant vertical deviation, as is typical of off shore or deep water drilling operations or when a horizontal well is desired. In such highly deviated holes, the torque and drag on the drill string are significant because the drill pipe lies against the low side of the hole. Under these conditions, the risk of pipe sticking is high when water based muds are used. In contrast oil based muds provide a thin, slick filter cake which helps to prevent pipe sticking. Thus the use of the oil based mud is preferred.
Despite the above noted benefits, the use of oil based drilling fluids and muds has it drawbacks. The initial start up and operational costs are often greater than drilling with a water based mud. However, often the higher costs can be justified if the oil based drilling fluid prevents the caving in or enlargement of the well bore which can greatly increase drilling time and well completion costs.
Environmental regulations have also severely limited the use of oil based and invert emulsion oil muds as drilling fluids. Of particular concern is the disposal of oil coated drilling cuttings especially in off shore or deep water drilling operations. Environmentally sound disposal of oil-coated cuttings is required which in these latter cases, the cuttings must be either processed onboard the drilling rig, or the cuttings must be shipped back to shore for disposal in an environmentally safe manner.
In view of the above, there exists a need for an oil based drilling fluid or invert mud emulsion that is soluble in sea water, especially cold sea water, yet stable under down-hole drilling conditions. Such a fluid can reduce the environmental impact of drilled cuttings discharged to the sea floor.